The present invention relates to the liquid crystal arts. It particularly relates to liquid crystal modulators for applications such as optical waveguides, optical beam scanners, computer-generated holograms, adaptive optics, flat-panel liquid crystal displays, and the like. However, the invention will also find application in conjunction with other electro-optical devices employing liquid crystals.
Liquid crystal materials include anisotropic molecules that can be electrically or magnetically aligned, tilted, rotated, or otherwise manipulated. For example, in nematic liquid crystal phases, the long axis of the anisotropic molecules are generally aligned, but there is substantially no translational ordering. In smectic liquid crystal phases, the long axis of the anisotropic molecules are generally aligned, and additionally some translational ordering is present.
In chiral or cholesteric liquid crystal phases, a mesoscopic helical ordering is also present. The long axis alignment of molecules is relatively rotated between successive layers of molecules, creating a helical rotation along a helix direction that is generally transverse to said layers. A helical pitch is defined as the average distance along the helix direction over which the relative rotation of successive layers corresponds to a complete 360° helical rotation of the anisotropic molecules. For short helical pitches of around 0.2–0.5 micron, the helical characteristic optically manifests as uniaxial birefringence with an optic axis along the helix direction.
In one typical electro-optic device, liquid crystal material is disposed between spaced apart confining surfaces, such as glass surfaces, at least one of which is light transmissive. For light transmission applications, both spaced apart confining surfaces are light transmissive. To induce lateral ordering of the anisotropic liquid crystal molecules, one or both confining surfaces are preferably anisotropic surfaces. For example, a thin polymer coating such as a polymide can be spread on the exemplary glass substrate and rubbed in a selected direction to provide a suitable anisotropic surface for inducing substantial lateral ordering of the liquid crystal molecules.
In the case of short-pitch chiral or cholesteric liquid crystal devices, the helix direction can be aligned perpendicular to the confining surfaces (called a Gradjean texture) or can be aligned parallel to the confining surfaces (called a fingerprint texture). Typically, the fingerprint texture has the helix axes randomly distributed with no long-range ordering in the plane of the confining surface. However, in electro-optic devices the rubbed or otherwise anisotropic surface preferably induces a uniformly lying helix texture in which the helix axis lies parallel to the confining surfaces along a selected helix alignment direction.
The uniformly lying helix texture is optically uniaxially birefringent with an optic axis along the helix alignment direction. In some liquid crystal materials arranged in a uniformly lying helix texture, application of a relatively small electric field directed perpendicular to the confining surfaces induces a flexoelectric in-plane deviation of the optical axis. For higher electric fields, dielectric coupling is present and the helix begins to unwind. At sufficiently high electric field, the helix is substantially completely unwound, producing a homeotropic texture.
For electro-optic device applications, a problem arises in that the unwinding of the helix is generally not completely reversible, and can produce degradation of alignment by the confining surfaces since either the planar or homeotropic surface condition supports the unwound helical structure in the uniformly lying helix texture, and the like. As a result, the electro-optic device does not fully return to the uniformly lying helix texture after the applied electric field is removed.
To address such problems, polymer stabilization of the uniform lying helix texture has been employed. In one configuration described in P. Rudquist, L. Komitov, and S. T. Lagerwall, Liquid Crystals volume 24, page 329 (1988), 10 wt % of a photoreactive monomer dissolved into a cholesteric liquid crystal was optically polymerized to produce a stabilizing polymer network which provided effective stabilization of the uniformly lying helix texture. However, the polymer network produced substantial residual birefringence that decreased cell contrast in the unwound state. This residual birefringence substantially decreased contrast and modulation efficiency and degraded switching time of electro-optic switching devices.
The present invention contemplates an improved apparatus and method which overcomes the aforementioned limitations and others.